Some"Best" Leicas from my personal experience:
M2: 35mm Summilux, Summicron, or Summaron; any 50mm; 90mm Elmarit (or Elmar)
M3: 50mm Noctilux, 75mm Summilux plus external finder; 90mm Summicron; 135mm
M4: same as M2
M5: any 35mm; 50mm f/1.4 or f/2; 90mm Elmarit
M6 with 0.58 finder: Any 28mm or 35mm for eyeglass wearers
M7 or MP with 0.58 finder: see M6 with 0.58 finder
Best small size Leica: The IIIc with Voigtlander lens and external finder
This is just me, now, OK?
May the light be with you.